This invention relates generally to apparatus and methods for measuring flow rates of fluids. In particular, the invention relates to an improved vortex flowmeter for ultra-pure applications. Such applications require that the flow-meter not introduce contaminants to the fluid flow being measured.
Vortex flowmeters measure the rate of flow of a fluid, termed a process fluid, by measuring the frequency of artificially induced vortices in the fluid. Such flowmeters are known in the art, and include those marketed by The Foxboro Company, Foxboro, Mass., U.S.A ("Foxboro"), for example, under the trade designation 83. Another example of a vortex flow meter known in the art can be found in U.S. Pat. No. 4,220,046, herein incorporated by reference. Vortex flowmeters are popular because of their relatively high accuracy and wide dynamic range. In addition, many vortex flow meters can operate in extreme temperatures, for example up to 800.degree. F. The term "ultrapure applications" herein refers to applications in which the process fluid has a purity of typically in the order of one part per trillion ("PPT").
Vortex flowmeters typically have a tubular passage, such as a pipe, for guiding the process fluid, and have a vortex shedder, also termed a bluff body, interposed in the path of fluid flow. The vortex-shedder creates a series of spaced vortices downstream in the flowing fluid. Under certain conditions, the vortex shedder creates two nearly-parallel rows of spaced vortices on opposite sides of the shedder. These vortices are known in the art as a Von Karman vortex street. The vortices in one row are staggered with respect to the vortices in the other row. It is understood that the frequency of these generated vortices is linearly proportional to the average flow velocity of the fluid. Thus, a measurement of the frequency of the vortices provides a measure of the average flow velocity. A vortex-responsive sensor detects the pressure fluctuations associated with the passage of the vortices and drives an electronic unit that determines the frequency of the vortices, to determine the flow velocity of the fluid.
The process fluid contacts many exposed surfaces of the vortex flow meter, such as surfaces of the tubular passage, of the vortex shedder, and of the sensor, as it passes through the instrument. In applications where the process fluid is ultra-pure, there is a risk of contamination of the process fluid as a result of such contact of the process fluid with fluid accessible surfaces of the flow meter.
It is known in the art to polish the fluid accessible surfaces of a vortex flowmeter to a surface smoothness typically of at least a No. 4 mill finish. Such smoothness of the fluid accessible surfaces diminishes entrapment of the process fluid in the surfaces, and diminishes contamination of the fluid as a result of the fluid removing particles from surfaces of the instrument. In addition to the described degree of smoothness of the surfaces, traditionally all inside angles between the various fluid accessible surfaces of a vortex flow meter are configured to be either greater than 135 degrees or rounded with a radius of curvature of at least 0.25 inches (0.635 cm).
It is costly and time-consuming to attain these stringent requirements regarding the smoothness of the surfaces and regarding the angles between the surfaces of a traditional vortex flowmeter for sanitary applications. Another disadvantage of polishing the fluid accessible surfaces of a vortex flow meter to the degree necessary for ultra-pure applications is that such a polishing typically results in some distortion of the contours of the vortex shedder. Such distortions typically degrade the accuracy of the measurement of the flow velocity of the fluid.
It is thus desirable to provide a vortex flow meter that meets the requirements for ultra-pure applications, and that is easier and less costly to produce than present vortex flow meters for ultra-pure applications.
It is another object of the invention to provide a vortex flow meter for ultra-pure applications that is at least equally as accurate as vortex flow-meters for other applications.
It is another object of the invention to provide a vortex flowmeter for ultra-pure applications that does not require a smoothness of its fluid accessible surfaces to the degree presently deemed necessary.
It is yet another object of the invention to provide a flowmeter whose fluid accessible surfaces do not entrap the process fluid.